Metal containers are used for storage of many different materials in industrial and retail consumer product containment applications. Well known examples of consumer metal storage container utilities include food cans, aerosol spray product cans and the like. Polyester films of various compositions are commonly laminated onto the can metal surface to provide a protective barrier against corrosive environmental conditions. These barrier films are typically laminated onto both sides of the metal so that the inner and outer side metal surfaces of the container are protected from degradation by the contained material and the external ambient environment, respectively. In the case of food containers, after containers are filled, they are exposed to a retort process that typically involves the use of live steam to sterilize the can and pasteurize or partially cook the contents. It is thus very desirable that the plastic and metal laminate structure of the container be able to withstand direct steam contact during such retort process conditions and have sufficient resistance to chemicals and chemically aggressive foods that may be stored in containers made from metal and film laminates.
Retort processing of plastic laminated metal containers can adversely affect the quality of the container. An area of particular interest is a visually observable defect on the outer side of metal containers that manifests as discolored or cloudy spots or areas. In the canning industry this defect is known as “blushing”. In the case of plastic film laminated on metal for canned goods, blushing can be attributed to accelerated hydrolysis of the film surface on the outer side of the container promoted by exposure to condensing steam during the retort process.
Another area of particular interest is the chemical resistance of the polymeric film laminated to the inner side of the container. This film can degrade because of contact during the long term storage of harsh chemicals and highly aggressive foods within the containers. Aggressive foods and chemicals can be corrosive to the container metal due to their proximity to ends of the pH scale, especially acidic materials having pH below about 3, and basic materials having pH above about 11. Aggressive foods may be acidic in nature and harsh chemicals may be basic in nature. Some examples of aggressive foods are beets, sauerkraut, and tomato products. Aggressive chemicals such as tub & tile cleaners may have a pH greater than 12.
Much attention has been devoted to the art of blush prevention of metal cans that use plastic film laminates on the outer side of the can. A preferred film composition is polyethylene terephthalate (“PET”). One of the most common methods cited to improve blush resistance uses polybutylene terephthalate (“PBT”) blended with PET in at least the outermost surface of the film. In addition, it has been cited that the blend should be at least 60 wt % PBT for effective blushing prevention. However, it has been discovered that films containing high levels of PBT blends in the outer layer or layers do not completely eliminate the blush formation problem. Thus there is a need in the canning industry for a polymeric lamination film that exhibits improved blushing resistance and excellent resistance to harsh chemicals and aggressive foods being stored in these containers. A need exists for polyester films for a protective barrier on the outer side of a metal can that will withstand the rigorous conditions of retort processing without blushing. In addition, it remains desirable to have polyester films to provide a protective barrier on the inside of a metal can that is able to withstand assault by aggressive foods and harsh chemicals during long term storage.